1. Field of the Invention
This application is the U.S. national phase of international application PCT/GB2004/004674, filed in English on 4 Nov. 2004, which designated the U.S. PCT/GB2004/004674 claims priority to GB Application No. 0325989.2 filed 7 Nov. 2003 and EP Application No. 03257035.0 filed 7 Nov. 2003. The entire contents of these applications are incorporated herein by reference.
This invention relates to a system for increasing the precision of a position sensor. In particular, this invention relates to a system for increasing the precision of a position sensing system incorporating magnetic field sensing devices such as Hall Effect devices.
2. Discussion of Prior Art
Position sensors and position sensing systems are often used in manufacturing applications where it is necessary to drill holes ‘blind’, that is where it is not possible to see the structure through which the hole is to be drilled. One example of such a manufacturing application is found in the aerospace industry when assembling a wing skin and a wing box where it is essential to determine accurately where to drill attachment holes through the wing skin and into the supporting feet of a rib of the wing box. Erroneous drilling of such attachment holes results in the incorrect distribution of stresses and strains through the airframe.
Conventionally, pilot holes are drilled from within the wing box outwards through the rib foot and the wing skin, a process known as back drilling. The pilot holes are then used as a guide for the drilling of attachment holes. However, since space is restricted within the wing box, the back drilling process is complex and prone to errors. Correction of these errors is a further time consuming task, and requires further drilling that may deleteriously effect the robustness of the resulting structure. There is thus a need for a position sensor to locate the correct drilling location for the attachment holes from without the wing box.
A known position sensor comprises an array of Hall Effect devices used to sense the magnetic field due to a cylindrical magnetic object placed at the desired location of the attachment hole. The Hall Effect devices are arranged in concentric circles such that the axis of the cylindrical magnetic object is located when each Hall Effect device in a given circle senses the same magnetic field. Such a position sensor is able from above the wing skin, to locate the axis of a cylindrical magnetic object placed on a rib foot within the wing box to within a tolerance of ±2.5 mm.
WO2004/016380 also describes a method and apparatus for locating non-visible objects. In this known method and apparatus the position of the object can be sensed by means of a suitable array of Hall effect sensors which can be moved relative to the object in question.
However, a greater degree of precision in locating the magnetic object is typically required for such position sensors to be useful in the aerospace industry or other industries where high precision during manufacture is required.